CS61A Lecture 13 Object-Oriented Programming

1. CS61A Lecture 13Object-Oriented Programming JomMagrotkerUC Berkeley EECS July 10, 2012

2. Thoughts on the Midterm? Using thumbs up/thumbs down... What did everyone think of the midterm?

3. Computer Science in the News http://www.computerworlduk.com/news/it-business/3368640/algorithmic-pricing-on-amazon-could-spark-flash-crash/

4. Today • Review: Binary Search Trees • Demo: Project 2 • Object-Oriented Programming • Defining our own data types! • Data with functions! • State!

5. Review: Binary Trees 1 Trees where each node has at most two children are known as binary trees. 2 5 3 4 6

6. Review: Binary Search Trees 10 Binary search trees are binary trees where all of the items to the left of a node are smaller, and the items to the right are larger. 4 12 2 6 14

7. Review: BST ADT empty_bst = None defmake_bst(datum, left=empty_bst, right=empty_bst): return (left, datum, right) defbst_datum(b): return b[1] defbst_left(b): return b[0] defbst_right(b): return b[2] Constructor Selectors

8. Review: Searching with BSTs defbst_find(b, item): if b == empty_bst: return False elifbst_datum(b) == item: return True elifbst_datum(b) > item: return bst_find(bst_left(b), item) return bst_find(bst_right(b), item)

9. Announcements • Homework 6 is due today, July 10. • Starting with the next homework, we will mark questions as core and reinforcement. • The core questions are ones that we suggest you work on to understand the idea better. • The reinforcement questions are extra problems that you can practice with, but are not as critical. • Homework 7 is due Saturday, July 14. • It will be released this afternoon. • Make sure you fill out a survey! • You must give us a completed survey to get back your exam once it has been graded.

10. Project 2 Demo Get started if you haven’t already!

11. Defining our Own Data Types So far, we’ve been defining new data types like this: defmake_account(owner, balance): “““Makes a bank account for the given owner with balance dollars ””” return (owner, balance) defaccount_balance(acct): “““Return acct’s balance””” return acct[1] defaccount_owner(acct): “““Return acct’s owner””” return acct[0] Constructor Can we group all of this into a single description of the data type? Selectors

12. Defining our Own Data Types Define the new data type Account class Account: def__init__(self, owner, balance): self.__owner = owner self.__balance = balance defget_owner(self): return self.__owner defget_balance(self): return self.__balance self refers to the specific instance of a class that we’re manipulating. Constructor Methods are operations that are associated with a class or object. Selectors Two underscores at the beginning of an instance variable means that the variable should only be accessed inside the class definition.

13. Object-Oriented Programming:Creating Objects >>> toms_account = Account(“Tom”, 50) We say that toms_account is an instance of the Account class. We call the process of creating an instance of a class instantiation.

14. Using Our New Data Type So how do we use it? >>> toms_account = Account(“Tom”, 50) >>> toms_account <__main__.Account object at 0x01...> >>> type(toms_account) <class ‘__main__.Account’> >>> Account.get_balance(toms_account) 50 >>> toms_account.get_balance() 50 >>> toms_account.get_owner() “Tom”

15. Dot Notation self.__balance toms_account.get_balance() “Dot Notation” is used for referring to the data that is associated with a specific object. Object Attribute As we saw on the previous slide, the dot notation also allows us to call an object’s associated functions without having to pass the object itself as the first argument.

16. Operating on our Data How would we have implemented withdraw and deposit operations before? defacct_withdraw(acct, amount): new_balance = acct.get_balance() - amount return Account(acct.get_owner(), new_balance) defacct_deposit(acct, amount): new_balance = acct.get_balance() + amount return Account(acct.get_owner(), new_balance) Can we associate the operations on our data type with the definition of the data type?

17. Operating on our Data class Account: def__init__(self, owner, balance): self.__owner = owner self.__balance = balance defget_owner(self): return self.__owner defget_balance(self): return self.__balance defwithdraw(self, amount): self.__balance -= amount defdeposit(self, amount): self.__balance += amount

18. Operating on our Data Using our operations for our accounts: >>> toms_account = Account(“Tom”, 50) >>> toms_account.get_balance() 50 >>> toms_account.withdraw(25) >>> toms_account.get_balance() 25 >>> toms_account.deposit(500) >>> toms_account.get_balance() 525

19. Object-Oriented Programming We now have a new programming paradigm! • Before – Functional Programming: • Think of our program as a series of functions. • Think in terms of inputs and outputs of each function. • There is no change over time, no state. • Now – Object-Oriented Programming: • Think of our program as a series of objects. • Think in terms of the ways in which objects interact with each other using methods. • Objects can change over time in our program. They have state.

20. Conclusion • Object-oriented programming: a brand new paradigm. • Preview: Classes that inherit traits from other classes and variables shared by all data of the same class.